This invention relates to a device for the recuperation of hydraulic energy and to a working machine with a corresponding device.
In the known interconnections of hydraulic cylinders in mobile working machines the retraction of hydraulic cylinders under pressing load (e.g. stroke of lowering without pressure) is realized by a throttle control. The potential energy which is defined by the load on the cylinder here is converted into heat by throttling the volume flow under pressure. Due to this process, the existing potential energy is destroyed. Due to the conversion into heat, additional cooling capacity furthermore must disadvantageously be applied within the machine.
A commonly used type of the hydraulic cylinders in mobile working machines is the differential cylinder. When the same is retracted by means of a throttle control and under pressing load, it must be ensured that refilling of the rod-side cylinder chamber is ensured. This is possible with the addition of a corresponding supply volume flow by the working pumps. Alternatively or in addition, a corresponding refilling of the rod-side cylinder chambers can be carried out by a recirculation of the throttled volume flow. By recirculating the throttled volume flow, a division of the volume flow corresponding to the area ratio of the hydraulic cylinders and corresponding to the volume ratio of the chambers of the hydraulic cylinder is made. A part of the volume flow here flows into the rod-side chambers of the cylinders and the other part is passed into the tank.
When the potential energy released during the lowering operation of the hoisting cylinders is to be stored, there is an interest in storing as much as possible of the existing energy. In hydraulics, this corresponds to the greatest possible volume flows under the highest possible pressure. The known hydraulic interconnections, which realize the recirculation of a part of the bottom-side volume flow into the rod-side chambers of the hydraulic cylinders, reduce the volume flow that can be available for storage.
At present, different solutions exist for the storage of the potential energy on lowering the boom of mobile hydraulic working machines.
The document US 2013/0081383 describes a solution in which one of two cylinders is used for storing energy. There is used a displacement machine in closed circuit, in order to refill the rod-side chambers of both cylinders with the return flow quantity of the second cylinder. A disadvantage of the invention described here is the non-existing exchange of oil on the bottom side of the hydraulic cylinder which is connected with the accumulator. The oil volume only is moved between hydraulic accumulator and bottom side of the cylinder.
Within an interconnection of document DE 10 2012 009 668 a hydraulic pump is utilized on retraction of the cylinders, in order to ensure refilling of the rod-side chambers. Refilling by application of hydraulic power does not correspond to an energy-efficient actuation of the hydraulic consumers.
What also is possible is the take-up of the potential energy of the boom by a gas-filled cylinder (DE 10 2010 051 665). In accordance with this invention the additional integration of a gas cylinder into the machine is necessary, which means a high integration expenditure.
At present, different possibilities exist for feeding in the stored hydraulic energy. The document WO 2013/180605 describes the direct feeding into the fan circuit of the machine. Based on the operating point of the fan circuit it is necessary to throttle the supplied volume flow from the hydraulic accumulator to the fan circuit. There are caused throttling losses and the amount of the reusable hydraulic energy thus is reduced.
It furthermore is possible to use the stored hydraulic energy directly for supplying the working pumps. This is described in the document DE 10 2005 052 108. There is required a circuitry which connects the suction side of the working pump either with the hydraulic tank or with the hydraulic accumulator. When the pump is not supplied via the hydraulic accumulator, pressure losses occur through the valve, which influence the intake pressure of the pump and thus can cause unfavorable operating conditions. In addition cooling and filtering must be provided between hydraulic accumulator and intake.
The known hydraulic interconnections correspondingly can have the following disadvantages:    1. The potential energy of the lifting-lowering operation is destroyed by the throttling operation and cannot be used for other processes.    2. The potential energy of the lifting-lowering operation is introduced into the hydraulic system in the form of thermal energy and must subsequently be discharged again by corresponding cooling devices. These processes likewise are consuming energy.    3. The division of the bottom-side volume flow on lowering of the hoisting cylinders leads to a reduction of the possible potential of storable energy.